1. Field of the Invention
The present invention relates to parking brake actuators that include mechanisms for automatically taking up slack in a brake cable of the parking brake system for vehicle.
2. Description of Related Art
In conventional parking brakes for vehicles, a hand or foot brake lever pivotally mounts to a frame of the vehicle or an intermediary parking brake actuator frame. A sheathed brake cable extends between an arm of the lever and a wheel brake of the vehicle. Pivotal movement of the brake lever from a disengaged position toward an engaged position tensions the brake cable, thereby engaging (i.e., activating) the wheel brake.
When a conventional parking brake is initially installed in a vehicle, slack in the brake cable is removed to minimize the extent to which the brake lever pivots before causing the wheel brake to begin engaging. Removing or taking up the slack in the brake cable minimizes the wasted stroke length of the lever and maximizes the useful lever stroke that an operator uses to create the braking force.
As a parking brake is repeatedly used, the brake cable tends to stretch and/or its cable sheathing tends to compress. This combination tends to create slack in the brake cable system that reduces the useful stroke length of the brake lever. In some circumstances the stretching of the cable and compression of the sheathing is so severe that the wheel brake does not securely engage even when an operator moves the brake lever into a fully engaged position.
Various conventional parking brake actuators include self-adjusting mechanisms that automatically take up slack in the brake cable to minimize or eliminate the problems associated with brake cable stretching. U.S. Pat. No. 5,950,496 discloses one such self-adjusting mechanism. A spring-biased tie rod extends between a lever-side end of the brake cable and a frame to automatically take up slack in the brake cable when the brake lever is disengaged and the brake lever is disconnected from the tie rod. As the brake lever is moved from its disengaged position toward its engaged position, clamping pieces, which attach to the brake lever, clamp and connect the tie rod to the brake lever such that additional movement of the brake lever moves the tie rod and brake cable in an engaging direction that causes the wheel brake to engage. When the brake lever is returned to its disengaged position, the clamping pieces release the tie rod to allow the spring biased tie rod to take up any slack that may have developed in brake cable.
Unfortunately, self-adjusting parking brakes such as that disclosed in U.S. Pat. No. 5,950,496 are not always reliable. For example, if the lever's clamping pieces fail to securely grasp the tie rod, then the brake lever will move into an engaged position without moving the brake cable (and wheel brake) into a fully engaged position. Consequently, a vehicle operator may believe, based on the engaged position of the brake lever, that the parking brake is fully and securely engaged, while in fact the wheel brake is not fully or securely engaged. U.S. Pat. No. 5,950,496 discloses the use of teeth on the tie rod and clamping pieces to help the clamping pieces securely grasp the tie rod. Unfortunately, teeth misalignment may cause only the tips of the mating teeth to contact each other when the lever moves toward the engaged position. The tip-to-tip contact may cause the teeth to slip relative to each other and cause subsequent teeth to bounce over each other without having the clamping pieces securely engage the tie rod.